Digital television systems used for terrestrial or satellite broadcast modulate and encode television signals for transmission by different methods and in different signal formats. The particular method and format adopted may be prescribed by an internationally recognized specification. One such specification, prepared for a European satellite communications system, is the "Specification of the Baseline Modulation/Channel Coding System for Digital Multi-Programme Television by Satellite" by the European Broadcasting Union, 19th Nov. 1993. This system is also known as the Direct Video Broadcast (DVB) system and covers both satellite and cable television signal distribution. Another transmission system, already in use in the United States and defined by a proprietary commercial specification, is the Digital Satellite System (DSS). However, whether the transmitted signal format is prescribed by a recognized standard or a proprietary commercial specification, a video signal receiver must be capable of receiving the transmitted signal format. A system for receiving different transmitted signal formats in the context of different types of transmission such as satellite, terrestrial and cable transmission is disclosed in U.S. patent application Ser. No. 08/342,280, Nov. 18th, 1994, entitled "A Branch Metric Computer for a Viterbi Decoder of a Punctured and Pragmatic Trellis Code Convolutional Decoder Suitable For Use in a Multi-Channel Receiver of Satellite, Terrestrial and Cable Transmitted FEC Compressed-Digital Television Data" by J. S. Stewart et al.
A video signal receiver employs demodulation and decoding functions that are specifically related to the signal format to be received. The demodulation function depends on the type of modulation, the signal shape, the data rate employed by the transmission system, and whether a single or differential output is required. The decoding function depends on the type of encoding, scrambling, interleaving and the code rate employed by the transmission system encoder.